# Concentrating acetic acid from vinegar

Vinegar seems to kill poison ivy or at least severely stunt their leaves. I am trying to increase the concentration of acetic acid and I put a bottle of vinegar in the freezer. I have looked at it and it has turned cloudy. I'm thinking if I poured this through a coffee filter I should be filtering out solid acetic acid. I might do this in the freezer because it might take some time. Does anyone think this will work? I might be getting water ice instead. Either way I would be separating the two. I could repeat this step and concentrate it further. Would I not really be filtering out anything? On another blog someone suggested buying aluminum acetate which has more acetic in it instead, but this might not be as effective.

• Yes, you are freezing out acetic acid, which has a melting point of 17C and your freezer is probably on the order of -10C. Your plan to filter it should give you very concentrated acetic acid, which could be a safety hazard. I would read this and this before planning how to proceed. You will probably want to dilute the acetic acid by about 3 parts water to one part AA. Note that this is not going to be an efficient solution and some AA will be left unfrozen in solution. – airhuff Apr 23 '17 at 18:08
• Thank you for your answer. I had bought a small bottle of lab grade acetic acid once which was pretty high %. I have experience working with this. Besides my throat has gotten more sensitive to things than it once was. I know to keep stuff like this at a distance. I used to keep the bottle in the bottom cabinet of a camper trailer and it would get down to the 20s sometimes. With that concentration it would look like it turned to gel or a chunk of ice in liquid at much higher temperature than that-much like the name glacial acetic acid. – discountbrainsurgery Apr 23 '17 at 19:12

If I understand correctly, what you described can be seen as a form of fractional freezing.

This is done in order to make alcoholic beverages more alcoholic (Eisbock is an example): the beverage is left to partially freeze and the ice is removed. Since ethanol has a lower melting point (-114 °C) than water (0 °C), this effectively removes more water than ethanol.

But in your case, since acetic acid has a higher melting point (16 °C) than water, the solids will have a higher concentration of acetic acid and you would want to keep that instead of the liquid.

Yes, you are freezing out acetic acid, which has a melting point of $\pu{17^oC}$ and your freezer is probably on the order of $\pu{-10^oC}$.

Your plan to filter the solid acetic acid should give you very concentrated acetic acid, which could be a safety hazard. I would read this and this before planning how to proceed. You will probably want to dilute the acetic acid by roughly 3 parts water to one part acetic acid while it is still solid, or wile it is melting to a slush. Note that this may not be a very efficient solution and some acetic acid will be left unfrozen (dissolved) in the original solution.

Regarding your overall goal to kill poison ivy:
You can greatly increase the contact area of the solution with the plant by adding some detergent, say dish-washing soap. Additionally, you can increase the toxicity of your solution to the target area in an environmentally friendly manner by dissolving a couple cups (sorry for the non-SI units, it's just how I do it) of table salt per gallon of the acetic acid solution. This will result in "poisoning" the area of the soil at the base of the plant, inhibiting it's ability to survive your chemical attack. You may find that a commercial 10% acetic acid solution is effective when used in this manner.

• Your hint of adding detergent to increase the permeability of acetic acid across the leaves brings back to my mind this recent publication: "Demonstrating the Effect of Surfactant on Water Retention of Waxy Leaf Surfaces" by Chiu et al. (J. Chem. Educ., 2017, 94, 230-234, DOI: 10.1021/acs.jchemed.6b00546). – Buttonwood Apr 23 '17 at 22:44
• I thought of adding detergent, but thought it might have some constituents that are basic and would neutralize the acid somewhat. I've heard of salt. I kinda didn't want to create a buildup of that in the soil. I know its probably negligible though. I've added linseed oil and turpentine. These will form a film on the leaves. – discountbrainsurgery Apr 23 '17 at 23:05
• @discountbrainsurgery , the effect of the small amount of detergent that you would need to get the effect is negligible compared to the amount of acetic acid present, but it sounds like you have that effect covered with your creative use of those oils. We just started adding salt to the solution this spring, so I can't speak directly to problems of long-term buildup. Partly it depends on the rainfall amount where you live, but sodium chloride is so water soluble it shouldn't accumulate much from season to season. I will say it seems to work much better on our weeds with the salt added. – airhuff Apr 23 '17 at 23:19
• I turned the nob up in my refrigerator to more than half way to max and found most of the contents of the bottle frozen. There was a little liquid inside. I poured this into a container. I have a wet bulb pH meter and I measured it. I got a pH of nearly 1.5. I don't think vinegar is anywhere near this. This is pretty acidic. Someone on stack exchange presented a phase graph showing the several regions depending on temp and % acetic acid where there is liquid water and solid AA and another where there is ice and liquid AA. – discountbrainsurgery Apr 24 '17 at 0:17
• I looked up the pH of regular vinegar and found it is typically 2.5. The pH I measured from what I poured out was 1.5 and still dropping. That's a whole factor of 10 or this solution is 10 times stronger. So, from 5% vinegar this is 50% acetic acid. If I repeat and separate this from the remaining liquid I doubt I'll get as strong a concentration which is already established knowledge. – discountbrainsurgery Apr 25 '17 at 0:43